[llvm] [NFC][Offload] Clarify `olDestroyQueue` (PR #152132)

[Ross Brunton via llvm-commits]

RossBrunton wrote:

This is a pure documentation update, it doesn't actually implement the logic it discusses (we currently always leak the Queue's internal data structures). I'm just creating this so we can discuss whether this is how the function should work.

Actually implementing it will probably require something like `olEnqueueCallback` to enqueue a callback at the end of the queue to clean it up.

https://github.com/llvm/llvm-project/pull/152132